[caver]

"The particular mix of strontium in the teeth of the four, the researchers concluded, showed that they were born and spent their early years in West Africa."

How do they know that they spent their early years in Africa? They could have already been here and filed their teeth here.

[Pharmboy]

"At least 10 skeletons appeared to be African, the researchers reported, and four had teeth with "unusually high" combinations of two isotopes of the element strontium. An isotope is a slight variation of a chemical element, with a different mass but otherwise the same as the basic element.

"In this case, the ratios of the isotopes strontium 87 and strontium 86 were consistent with those in the teeth and bones of people who were born and grew up in West Africa. A comparison with strontium measurements of people born in Mesoamerica showed no similarities with the four specimens."

[Robert A Cook PE]

Filed teeth was a characteristic of West African born youth, not central American people.

[mewzilla]

FWIW, from this this link:

Strontium isotopes in human teeth and bones as a monitor of mobility

Direct evidence of human residential mobility has been difficult to obtain in prehistory. Archaeologists have generally used artifact styles as a proxy for people to investigate population movement. We approach this problem by comparing strontium isotope ratios in human bones and in teeth . Dental enamel forms during early childhood and retains Sr isotopes from the local geology at that time, while bone, which is chemically remodeled, assimilates Sr isotopes reflecting residence during the last years of one's life. Thus differences in strontium isotope ratios between bone and tooth enamel reveal movement during the life of the individual. Strontium isotope ratios in teeth that do not match those of the local geology should indicate immigrants to a site...

You can get the rest from the link if you like :)

[doc30]

Strontium is a trace element whose concentrations vary with respect to geographic location. The amount of strontium in Mexico is different than in Africa. While your teeth are developing, traces of strontium are included. The concentration in teeth parallels the concentration in the environment where the teeth developed. The African skeletons found in Mexico had strontium concentrations similar to those found in people from sub Saharan Africa, not from Mexico. This is fairly conclusive proof these people were born and grew up in Africa.

[adam_az]

"How do they know that they spent their early years in Africa? They could have already been here and filed their teeth here."

chemical signatures of their teeth

[elli1]

How do they know that they spent their early years in Africa?

Because the stontium was incorporated into their teeth when they were children in Africa.